Is it possible to have a race condition for a write-only operation?

Question

If I have several threads trying to write the same value to a single location in memory, is it possible to have a race condition? Can the data somehow get corrupted during the writes? There is no preceding read or test conditions, only the write...

EDIT: To clarify, I'm computing a dot product on a GPU. I'm using several threads to calculate the individual products (one thread per row/column element) and saving them to a temporary location in memory. I need to then sum those intermediate products and save the result.

I was thinking about having all threads individually perform this sum/store operation since branching on a GPU can hurt performance. (You would think it should take the same amount of time for the sum/store whether it's done by a single thread or all threads, but I've tested this and there is a small performance hit.) All threads will get the same sum, but I'm concerned about a race condition when they each try to write their answer to the same location in memory. In the limited testing I've done, everything seems fine, but I'm still nervous...

Answer 1

Under most threading standards on most platforms, this is simply prohibited or undefined. That is, you are not allowed to do it and if you do, anything can happen.

High-level language compilers like those for C and C++ are free to optimize code based on the assumption that you will not do anything you are not allowed to do. So a "write-only" operation may turn out to be no such thing. If you write i = 1; in C or C++, the compiler is free to generate the same code as if you wrote i = 0; i++;. Similarly confounding optimizations really do occur in the real world.

Instead, follow the rules for whatever threading model you are using to use appropriate synchronization primitives. If your platform provides them, use appropriate atomic operations.